The present invention relates to an RF (radio frequency) coil, shield and a magnetic resonance imaging apparatus, and particularly relates to a TEM (transverse electromagnetic mode) resonator RF coil, a shield used to adjust TEM RF coils, and a magnetic resonance imaging apparatus using the TEM resonator RF coil.
In magnetic resonance imaging apparatus using for example a high magnetic field of approximately 3 T (tesla) as the static magnetic field, TEM resonator RF coils are utilized on account of a high RF (radio frequency) signal transmit/receive efficiency to receive magnetic resonance signals occurring from spin excitation and RF excitation of the spin of the imaging target.
The TEM resonator RF coil has a cylindrical tube 700 as shown in FIG. 1. Both ends of the tube 700 have orifices 702, 702xe2x80x2. The orifices 702, 702xe2x80x2 have openings 704, 704xe2x80x2 concentrically formed with a smaller inner diameter than the inner diameter of the tube 700.
The tube 700 and the orifices 702, 702xe2x80x2 are formed with consecutive conductive elements. Tube bodies having a tube 700 and orifice 702, 702xe2x80x2 of this kinds are called shields or cavities.
A plurality of line elements 802 are disposed on the inner side of the shield in parallel with the shield axis. The line elements 802 form an LC series circuit. Both ends of the plural line elements 802 are respectively coupled mechanically and electrically to the orifices 702, 702xe2x80x2, and arranged concentrically along the periphery of the openings 704, 704xe2x80x2. The line elements 802 are separated from the inner side of the tube 700.
A rotating RF magnetic field is generated within the surface axially perpendicular to the columnar space enclosed by the line elements 802, by supplying an RF signal to specified locations of an RF coil of this structure. The RF signal (magnetic resonance signal) generated by the rotating spin on the same plane is extracted from specified positions of this RF coil.
In actual magnetic resonance imaging, the shield is separated into a plurality of slits 706 in the axial and radial directions as shown for example in FIG. 2. Each of the shield segments separated by the slits 706 are electrically isolated, preventing excess current from flowing along the outer periphery of the shield during application of a magnetic field gradient, and preventing disturbances in the static magnetic field during excess current flow.
In the RF coil having the shield separated by slits, the shield effect is decreased because both ends of the orifices 702, 702xe2x80x2 are separated into individual segments by the slits 706. When this kind of RF coil is used as the head coil for capturing images of the cranium, the neck section of the target image is added as an external load to the other orifice so that the electrical characteristics are asymmetrical along the coil axis and the RF coil operation tends to be unstable.
To achieve stable operation, capacitors 122 are disposed to connect the adjoining shield segments on both ends of the orifices 702, 702xe2x80x2 of the shield as shown in FIG. 3.
Capacitors used as the capacitor 122 have for example, sufficient high impedance in a frequency region of 1 kilohertz to 10 kilohertz and for example, sufficiently low impedance in the frequency range of approximately 128 megahertz.
The orifices 702, 702xe2x80x2 connected by such capacitors are equivalent to an electrical short in the RF region across the slit 706. A plurality of the line elements 802 therefore have a value equivalent to conductive elements each jointly connected on both ends, so the shield effect in the RF region is improved, and operation is stabilized regardless of whether a load is present or absent in the vicinity of the orifices 702, 702xe2x80x2.
In the 1 kilohertz to 10 kilohertz frequency region on the other hand, the capacitor 122 has a sufficiently high impedance so that excess current does not flow to the outer periphery during application of a magnetic field gradient forming the signal in the same frequency range. Therefore the effect of excess current on the static magnetic field can be prevented.
The RF coil imaging object can be easily input and output so that a two-segment structure along the axis can be formed as shown for example in FIG. 4, and both segments joined by a connector. In such a case, the width of the slit 706 in the connector joint must be wider than the other sections so that the static electrical coupling in that section is weak and the shield effect deteriorates.
In the joint of the connector 124, the slits 706, 706xe2x80x2 are respectively rotated into both shield segment sides as shown for example in FIG. 5, conductive foil lands 126, 126xe2x80x2 are respectively formed, and electrically connected to the connector 124, and the slits 706, 706xe2x80x2 respectively bridged to the capacitors 122, 122xe2x80x2. In this way a sufficient static electrical connection can be maintained in the segments.
In the above segmented TEM resonator RF coil, the conductive pattern for electrical connection to the connectors is different from the conductive pattern of other RF coils so that the circuits in the segments have poor uniformity, and the operating characteristics as an RF coil deteriorate.
It is an object of the present invention to provide a TEM resonator RF coil with excellent operating characteristics, a shield for adjusting this type of RF coil, and a magnetic resonance imaging apparatus using this TEM resonator RF coil.
(1) To solve the above problems, in a first aspect of the invention, a TEM resonator RF coil is comprised of a cylindrical shield having a ring-shaped orifice at both ends, a plurality of line elements connected at both ends to the orifice and arranged at equal intervals along an opening of the orifice, a plurality of slits segmenting the shield into two equally portioned positions at line element intervals in parallel along the axis and forming a plurality of ring segments on the orifice, and the plural ring segments are comprised of a first conducting pattern connected to the line elements, a second conducting pattern disposed symmetrically for the line elements in two directions from the line elements towards the slit, capacitors connecting the first conducting pattern with the second conducting pattern, and a connection means to electrically connect the second conducting patterns spanning the adjacent ring segments.
In this aspect of the invention, the circuits between the segments are uniform (equivalent) because the plural ring sections made from an orifice segmented by slits, have a first conducting pattern connected to the line elements, a second conducting pattern disposed symmetrically to the first conducting pattern for the line elements, in two directions from the line elements towards the slit.
(2) In another aspect of the invention to solve the above problems, the RF coil according to (1) is characterized in that the shield can be divided into two portions at the slit locations.
In this aspect of the invention, the shield can be segmented into two portions at the slit locations so that the uniformity of the circuits is continually maintained between segments, and the imaging object can be easily inserted and removed.
(3) In another aspect of the invention to solve the above problems, the RF coil according to (1) is characterized in that the shield can be disassembled into a plurality of cylinder segments per the slit locations.
In this aspect of the invention, the shield can be disassembled into a plurality of cylinder segments at the slit locations so that the uniformity of the circuits is continually maintained between segments, and frequency alignment to the same frequency can be performed in each segment.
(4) In another aspect of the invention to solve the above problems, the TM resonator RF coil comprises a cylindrical shield having a ring-shaped orifice at both ends, a plurality of line elements connected at both ends to the orifice and arranged at equal intervals along the opening of the orifice, a plurality of slits segmenting the shield into two equally portioned positions at line element intervals in parallel along the axis and characterized in that the shield can be disassembled into a plurality of cylinder segments at the slit locations.
In this aspect of the invention, the shield can be disassembled into a plurality of cylinder segments at the slit locations so that frequency alignment to the same frequency can be performed in each segment.
(5) In another aspect of the invention to solve the above problems, the RF coil according to (2) or (4) is characterized in comprising a support means to support the plural cylinder segments so as to constitute a whole cylinder.
In this aspect of the invention, the plural cylinder segments are supported as a whole cylinder by the support means.
(6) In another aspect of the invention to solve the above problems, an RF coil according to (5) is characterized in that the support means can be divided into at least two portions in parallel along the axis.
In this aspect of the invention, the support means can be divided into two portions so the imaging object can be easily inserted and removed.
(7) In another aspect of the invention to solve the above problems, a shield includes a fit portion capable of receiving one cylinder segment for an RF coil according to (3) or (4).
In this aspect of the invention, during frequency alignment of the segments to the same frequency, the shield acquires a jig for fitting the segment into the fit portion.
(8) In another aspect of the invention to solve the above problems, the shield according to (7) coil is characterized in having slits corresponding to the slits in the RF coil according to (3) or (4).
In this aspect of the invention, the shield has slits corresponding to the slits in the RF coil and so is equivalent to the RF coil shield.
(9) In another aspect of the invention to solve the above problems, in a magnetic resonance imaging apparatus for forming images based on magnetic resonance signals acquired by applying a high frequency magnetic field to an object for imaging under a static magnetic field and a gradient magnetic field, an RF coil for at least either applying the high frequency magnetic field or acquiring the magnetic resonance signal is a TEM resonator coil comprised of a cylindrical shield having a ring-shaped orifice at both ends, a plurality of line elements connected at both ends to the orifice and arranged at equal intervals along the opening of the orifice, a plurality of slits segmenting the shield into two equally portioned positions at line element intervals in parallel along the axis and forming a plurality of ring segments on the orifice, and the magnetic resonance imaging apparatus is characterized in that the plural ring segments are respectively comprised of a first conducting pattern connected to the line elements, a second conducting pattern disposed symmetrically for the line elements in two directions from the line elements towards the slits, capacitors connecting the first conducting pattern with the second conducting pattern, and a connection means to electrically connect the second conducting patterns spanning the adjacent ring segments.
In this aspect of the invention, a plurality of ring segments on the orifice capable of being divided by the slits, respectively have a first conducting pattern connecting the line elements, and a second conducting pattern disposed symmetrically for the line elements in two directions from the line elements towards the slit, so that the uniformity of the circuits is continually maintained between segments. By utilizing an RF coil of this type, high quality imaging can be performed.
(10) In another aspect of the invention to solve the above problems, a magnetic resonance imaging apparatus according to (9) is characterized in that the shield can be segmented into two portions at the slit locations.
In this aspect of the invention, the shield can be segmented into two portions at the slit locations so that the uniformity of the circuits is continually maintained between segments, and the imaging object can be easily inserted and removed.
(11) In another aspect of the invention to solve the above problems, a magnetic resonance imaging apparatus according to (9) is characterized in that the shield can be disassembled into a plurality of cylinder segments at the slit locations.
In this aspect of the invention, the shield can be disassembled into a plurality of cylinder segments at the slit locations so that the uniformity of the circuits is continually maintained between segments, and frequency alignment to the same frequency can be performed in each segment. By utilizing an RF coil of this type, high quality imaging can be performed.
(12) In another aspect of the invention to solve the above problems, in a magnetic resonance imaging apparatus for forming images based on magnetic resonance signals acquired by applying a high frequency magnetic field to an object for imaging under a static magnetic field and a gradient magnetic field; an RF coil for at least applying the high frequency magnetic field or acquiring the magnetic resonance signal is a TEM resonator coil comprised of a cylindrical shield having a ring-shaped orifice at both ends, a plurality of line elements connected at both ends to the orifice and arranged at equal intervals along the opening of the orifice, a plurality of slits segmenting the shield into two equally portioned positions at line element intervals in parallel along the axis, and the magnetic resonance imaging apparatus is characterized in using an RF coil with a shield capable of being disassembled into a plurality of cylinder segments at the slit locations.
In this aspect of the invention, the shield can be disassembled into a plurality of cylinder segments at the slit locations so that frequency alignment to the same frequency can be performed in each segment. By utilizing an RF coil of this type, high quality imaging can be performed.
(13) In another aspect of the invention to solve the above problems, a magnetic resonance imaging apparatus according to (11) or (12) is characterized in comprising a support means to support the plural cylinder segments so as to constitute a whole cylinder.
In this aspect of the invention, a plurality of cylinder segments can therefore be supported together as a whole cylinder by the support means.
(14) In another aspect of the invention to solve the above problems, a magnetic resonance imaging apparatus according to (13) is characterized in that the support means can be divided along the axis into at least two portions.
In this aspect of the invention, the support means is divided into two portions so that the imaging object can be easily inserted and removed.
Therefore, the present invention provides a TEM resonator RF coil with excellent operating characteristics, a shield used to adjust the TEM RF coil, and a magnetic resonance imaging apparatus using the TEM resonator RF coil of the invention.
Further objects and advantages of the present invention will be apparent from the following description of the preferred embodiments of the invention as illustrated in the accompanying drawings.